The present invention relates to a combustion control system which can perform optimum combustion control with harmful exhaust gas reduced by detecting the combustion state of each cylinder on the basis of an ion current detected value to suppress a combustion change, and more particularly to a combustion control system which can realize the combustion control with high reliability with no increase in cost.
It is generally known that during combustion of an internal combustion engine, conductive radical composition is produced in a flame so that the combustion state can be detected in terms of an ion current.
Specifically, with an internal wall of an engine combustion chamber and an ignition plug in the chamber used as electrodes for detecting the ion current, if a prescribed voltage is applied between both electrodes (internal wall and ignition plug), the ion current flows between both electrodes when the flame in combustion passes between the electrodes.
Further, when the flame burns to extend, the radical density existing between both electrodes is apt to change in accordance with a combustion state. For this reason, it is also known that the detected value of the ion current changes according to the extension of flame leading to a change in the combustion state and also according to the shape of the flame. Therefore, using such a property of the ion current, the combustion state of the internal combustion engine can be detected.
FIG. 8 is a waveform chart showing the time change in an ion current Io which is detected during combustion in a cylinder.
As seen from FIG. 8, the ion current Io represents an output waveform having plural maximum values within a slight change. This output waveform occurs during the period of main flame propagation in the combustion flame in the internal combustion engine.
Commonly, ignition timing Ts is set at a more leading angle side than the crank angular position of TDC. At the time of ignition, combustion starts from the area in proximity to the ignition plug to generate ions. The ion current Io, therefore, has a maximum peak at the ignition time before TDC.
Further, the combustion expands at the crank angular position after TDC to provide a maximum quantity of ions at a portion separate from the ignition plug. Thus, the ion current Io reaches its maximum value.
Ions remain also during the period (several msec or so) after completion of combustion. Therefore, because of air movement within the combustion chamber, the detected value of the ion current Io changes to converge.
Also after the period of main flame propagation is completed, the fuel in a liquid state attached to the internal wall of the combustion chamber is vaporized so that a small flame called "after-burning" which does not contribute to an output torque is created. Correspondingly, the ion current Io is detected.
It is also known that several kinds of waveform processing for the detected value of the ion current Io provide a characteristic value greatly correlated with the combustion state.
A previously known technique of controlling an internal combustion engine using the above characteristic of the ion current Io is disclosed in e.g. JP-A-6-42384.
FIG. 9 is a schematic arrangement view of the internal combustion engine control system disclosed in the above publication. Reference numeral 100 denotes an internal combustion engine; 101 an air intake system for supplying fuel and air to the engine; 102 a throttle valve provided within the air intake system 101 to respond to an accelerator pedal (not shown); 103 a surge tank provided on the downstream side of the throttle valve; 104 an air intake manifold communicating with the surge tank 103; 105 a fuel injection valve on the downstream side of the air intake manifold 104.
Reference numeral 106 denotes a cylinder of the engine 100; 107 an ignition plug provided within the combustion chamber of the cylinder 106; and 108 and 109 an intake tube and an exhaust tube attached to the cylinder 106, respectively. The intake tube 108 opens/closes the cylinder on the side of the air intake system. Reference numeral 110 denotes an exhaust system communicating with the exhaust tube 109. The exhaust system opens/closes the cylinder 106 on the side of the intake system 101. Reference numeral 111 denotes a catalyst provided on the downstream side of the exhaust system 110 to clean the exhaust gas.
Reference numeral 112 denotes an air/fuel sensor for detecting the air/fuel ratio h on the basis of the oxygen density within the exhaust system 110; 113 a water temperature sensor for detecting the cooled water temperature e of the engine; 114 an idle switch for detecting the complete closed state of the throttle valve 102; 115 an intake air pressure sensor for detecting the intake air pressure a within the air intake system 101; and 116 a revolving speed sensor for detecting a revolving speed b of an engine; and 117 a vehicle speed sensor for detecting a vehicle speed c.
Reference numeral 10 denotes a vehicle-loaded electronic control unit (ECU) of a microcomputer. On the basis of the items of detected information a to f from the several kinds of sensors 112 to 117, the ECU 10 produces a fuel injection signal F (which serves to determine a fuel injection timing and a fuel injection quantity) for the fuel injection valve 105 and an ignition driving signal Q (which serves to determine the ignition time).
The ECU 10 includes a CPU 11 for performing several kinds of arithmetic processing on the detected items of information a to f from the several kinds of sensors, an RAM 12 serving as a memory during arithmetic processing in cooperation with the CPU 11, an input interface 13 for taking in the detected items of information a to f, and an output interface 14 for producing the fuel injection signal F and the ignition driving signal Q, whereby the several parameters of the engine are controlled by the arithmetic processing corresponding to a running condition.
Reference numeral 23 denotes a current backflow preventing diode for detecting an ion current connected to the ignition plug 107; and reference numeral 24 denotes a biasing power source for passing an ion current. These components constitutes an ion power source detecting means in cooperation with the ECU 10.
The ECU 10 also includes an ROM (not shown) storing an operation program previously. Using this operation program, the ECU 10 produces the fuel injection signal F and ignition driving signal Q optimal to a running condition on the basis of the outputs a to f from the several kinds of sensors to drive the injection fuel valve 105 and ignition plug 107.
The ECU 10 detects the combustion state such as a misfire on the basis of the detected value of the ion current Io, and stops the combustion injection signal F for detection for the misfire to make the control of cutting the combustion.
In FIG. 9, although the concrete construction of the ion current detecting means is not shown, it should be noted that when the ion current is passed through the biasing power source 24 and the backflow preventing diode 23, the detected value of the ion current Io is supplied to the ECU 10.
In the ion current detecting technique disclosed in JP-A-6-42384, the analog value of the ion current Io is A/D converted by the ECU 10 to provide a large quantity of digital data which is in turn stored in the RAM 12 of a large capacity of memory. In this technique, these digital data will be subjected to arithmetic processing.
Incidentally, an exemplary concrete construction of the system for detecting the ion current from the combustion chamber of a vehicle-loaded internal combustion engine is shown in a misfire detecting circuit disclosed in JP-A-2-104978.
In this device, however, the detected value of the ion current Io is given as an analog voltage. Therefore, in order to perform the signal processing accurately using a vehicle loaded digital computer, the analog signal must be A/D converted by sampling for each 1.degree. or so in terms of the crank angle of the internal combustion engine.
The large quantity of digital data thus A/D converted are once stored in the RAM 12, and are subjected to arithmetic processing at a high speed within a period until next combustion. However, this requires a very large amount of memory capacity. For this reason, it is difficult to perform this arithmetic processing by "one chip microcomputer" for controlling a vehicle loaded engine.
As described above, the conventional combustion control system for an internal combustion engine must process the large amount of digital data on the ion current detected value, and hence cannot realize the combustion control with high reliability.